1. Field of the Invention
The present invention relates to a color image forming apparatus and an image quality control system for a color copying machine, a color printer, and the like.
2. Description of the Related Art
Hitherto, there is a color image forming apparatus of a so-called inline-system, in which a plurality of photoreceptor drums for forming an image carrier are arranged in a row. This apparatus forms a color image by transferring toner images of yellow, magenta, cyan, and black sequentially and superimposing these colors on a sheet by means of four photoreceptor drums disposed along a traveling path of transfer material, while carrying and transferring the transfer material on an electrostatic image transfer belt tensed by a plurality of rollers.
Recently, attention is being given to such structure because of its high-speed printing capability.
However, since the respective colors are formed by four photoreceptor drums, a higher degree of accuracy is required in terms of rotation of the photoreceptor drum in comparison with a color image forming apparatus having such structure that four transfer paths are provided per color and one photoreceptor drum passes through these paths to superimpose a plurality of colors (hereinafter referred to simply as “four-path system”).
In other words, in general, the photoreceptor drum is driven by a gear train, and thus uneven rotation at low frequencies corresponding to the component of one revolution of the gear are inevitable. However, in the case of the four-path system, the speed reduction ratio of the drive gear train is set to the combination of integral numbers, and thus accumulated pitch errors of the gears can be avoided, which enables accurate positioning of image formation in each color.
However, in the case of the in-line system, since a plurality of photoreceptor drums are independently provided, the drive gear trains are also independently provided. Therefore, the method of avoidance as described above in conjunction with the four-path system can hardly be realized, and hence a degradation in image quality called color drift caused by shifting of image forming positions of the respective colors may often occur.
In the related art, countermeasures for color drift such as controlling the revolution of the motor to detect and cancel out the uneven speed by detecting the angular speed of the photoreceptor drum or reading the image transferred onto the transfer material, or reducing relative color drift by adjusting the rotational phase of the respective photoreceptor drum in a desirable state as disclosed in JP-A-9-146329 and JP-A-10-333398 has been taken.
However, in the examples in the related art described above, since the drive control of the image carrier forming the photoreceptor drum is a speed reduction system via a plurality of gears, the uneven speed appears in a complex speed profile including the component of one revolution cycle of the intermediate gear in addition to the component of one revolution cycle of the photoreceptor drum, and thus an uneven speed detecting unit and motor control with a high degree of accuracy are required. However, even when the phase of the photoreceptor drum is adjusted, an influence of the intermediate gear remains, and thus complex computation such as integration is required for detecting drive variations of the image carrier at every one revolution cycle, which places significant burden on the CPU and memory.